The technical field of this invention is vehicle systems incorporating vehicle wheel brakes, and particularly such systems for determining brake rotor temperature.
Braking performance can be significantly affected by the temperature rise in brake components such as rotors. For this reason, systems have been proposed for monitoring the temperature of brake components. For example, temperature sensors such as thermocouples have been used in brake system and/or component testing, although such sensors are not practical for mass-produced vehicle applications. In addition, brake component estimation algorithms have been proposed. Such monitoring allows the generation of a driver warning or even the automatic reduction of braking pressure of individual wheels in braking or traction control, as described in U.S. Pat. No. 5,136,508, issued Aug. 4, 1992.
But the algorithm of the aforementioned patent has shortcomings. First of all, it is a method for estimating brake lining temperature; but brake lining temperature is much less well defined than brake rotor temperature. The linings, in a brake caliper pad, for example, are made of a substance having a much lower heat conductivity (higher temperature insulating effect) and a higher wear rate than the same characteristics of a brake rotor. Corroboration of any temperature estimation algorithm requires an accurately measured temperature test using a temperature sensor such as a thermocouple. Mounted in a brake lining, such a thermocouple must be placed a significant distance away from the rotor/lining interface so that, even with lining wear, the sensor will not be contacted and compromised by the metallic, fast spinning rotor. In the low heat conductivity environment of a brake lining, the distance of the thermocouple from the rotor/lining interface will provide a large temperature insulation between the interface and the sensor that leads to inaccuracy in sensing of the interface temperature. But a metallic brake rotor has a high heat conductivity and a low wear rate, in comparison with a lining. This allows a rotor mounted, corroborating thermocouple to be placed much closer, comparatively, to the rotor/lining interface if it is incorporated in a brake rotor rather than a brake lining. It will thus provide a more accurate reading of the temperature of the rotor/lining interface during braking.
This effect is even more true in a dynamic temperature variation. As brake pressure varies and the temperature at the brake rotor/lining interface heats up and cools down, most (typically more than 95%) of the heat generated at the interface flows away through the highly conductive rotor. The temperature measured by the well insulated lining thermocouple will vary less and with much greater time delay. The temperature measured by the rotor mounted thermocouple will follow the dynamic variation of the temperature at the rotor/lining interface far better than the temperature measured by a lining mounted thermocouple. Thus, brake rotor temperature can be much more accurately defined with real time temperature measurement; and this provides a more accurate corroboration for a brake rotor temperature estimation algorithm than for a brake lining temperature algorithm. Since brake rotor temperature provides a more accurate indication of temperature at the rotor/lining interface, a temperature estimation algorithm can, and should, provide a more sophisticated, and thus more accurate, estimated value of the brake rotor temperature. For example, temperature effects due to fore and aft vehicle load shift due to braking and/or cooling effects during braking are significant and measurable.
The temperature estimation algorithm of the aforementioned patent estimates a vehicle speed dependent cooling effect on the brake lining, but only when the brakes are not being applied. In reality, the cooling effect occurs at all times; and this is especially true for a brake rotor, which radiates heat more efficiently, both because of its metal structure and because of the great portion of its surface area that is not covered by the brake pad/lining and is thus exposed directly to the air.
In addition, the algorithm of the aforementioned patent relies heavily on exponential functions, which greatly consume controller computer resources, especially memory. Thus, to be practical and accurate, a brake temperature estimation algorithm should preferably avoid the use of exponential and other memory intensive functions.
This invention is a method and apparatus for providing an estimated brake rotor temperature. It provides for a cooling effect during braking as well as when braking is inactive, the cooling effect being based on a difference between the rotor temperature and sensed ambient temperature and also preferably based on wheel speed. In an active braking mode, it provides for a heating effect based at least on sensed wheel speed. If no brake pressure signal is available, it further bases the active braking heating effect on vehicle deceleration. Preferably, it provides compensation between front and rear brake units for differences in heat generation due to load shifts during braking, the compensation being preferably based on vehicle deceleration. It also does not use computer resource hungry exponential functions in determining the estimated brake rotor temperature.